In wire and cable (W&C) applications a typical cable comprises a conductor surrounded by one or more layers of polymeric materials. The cables are commonly produced by extruding the layers on a conductor. One or more of said layers are often crosslinked to improve e.g. deformation resistance at elevated temperatures, as well as mechanical strength and/or chemical resistance, of the layer(s) of the cable.
Crosslinking of the polymers can be achieved e.g. by free radical reaction using irradiation or using a crosslinking agent which is a free radical generating agent; or via hydrolysable silane groups present in the polymer using a condensation catalyst in the presence of water.
Power cables are defined to be cables transferring energy operating at any voltage level. The voltage applied to a power cable can be alternating (AC), direct (DC) or transient (impulse). Moreover, power cables are typically indicated according to their level of operating voltage, e.g. a low voltage (LV), a medium voltage (MV), a high voltage (HV) or an extra high voltage (EHV) power cable, which terms are well known. Power cable is defined to be a cable transferring energy operating at any voltage level, typically operating at voltage higher than 100 V. LV power cable typically operates at voltages of below 3 kV. MV and HV power cables operate at higher voltage levels and in different applications than LV cables. A typical MV power cable usually operates at voltages from 3 to 36 kV, and a typical HV power cable at voltages higher than 36 kV. EHV power cable operates at voltages which are even higher than typically used for HV power cable applications. LV power cable and in some embodiment medium voltage (MV) power cables usually comprise an electric conductor which is coated with an insulation layer. Typically MV and HV power cables comprise a conductor surrounded at least by an inner semiconductive layer, an insulation layer and an outer semiconductive layer, and in that order.
Silane-crosslinked materials have been used primarily as insulation layer in low voltage cables and as insulation and semiconductive layer in medium and to some extent also for high voltage cables.
Moreover, in WO 2007/071274 the need for a cable to be flexible in different applications is discussed. It is disclosed that in order to be correctly installed with simple and quick operations, a cable needs to be particularly flexible so that it can be inserted into the wall passages and/or wall conduits and follow the bends of the installation path without being damaged. And, it is also disclosed that increasing the flexibility of an electric cable can reduce the damages to the cable during customer installation caused by tearing or scraping actions. Furthermore, it is disclosed that the flexibility of the cable can be increased by providing the cable with an expanded insulating layer, with favorable results in the installation process of the cable.
Further, in WO 2007/071274 the observation “that if expanding and cross-linking a polyolefin is attempted, the expansion degree cannot in general be controlled, being either excessive or insufficient” is mentioned.
And, accordingly, WO 2007/071274 relates to a process for manufacturing an electric cable comprising an insulating coating of an expanded and silane-crosslinked polyolefin material, wherein the process comprises forming a blend of a polyolefin material, a silane-based crosslinking system and a foaming system.
However, there is still need for a process for manufacturing a power cable having an expanded and crosslinked insulation layer where process parameters may be optimized without having to take a reactive silane grafting process into consideration during extrusion.